1. Field of the Invention
The present invention relates to an antilock brake liquid-pressure control apparatus, and more particularly to an antilock brake liquid-pressure control apparatus of the variable volume type which is small in size and low in cost, and does not need, for its construction, a high pressure accumulator, a pressure switch, a large-size reservoir tank, and the like.
2. Related Art
There has been known an antilock brake liquid-pressure control apparatus of the called variable volume type. An example of the antilock brake liquid-pressure control apparatus of this type is disclosed in Japanese Utility Model Publication No. Hei. 5-467. In the liquid-pressure control apparatus disclosed, a cut valve is provided at the midway in a main liquid path, which interconnects a master cylinder and a wheel cylinder. The cut valve is provided for cutting off the main liquid path. A liquid pressure control housing is located closer to the wheel cylinder than to the cut valve. A liquid-pressure control piston is slidably provided in the liquid pressure control housing. The inner space of the liquid pressure control housing is partitioned into a first liquid chamber and a second liquid chamber by the liquid-pressure control piston. The first liquid chamber communicates with the wheel cylinder and the cut valve, and the second liquid chamber is isolated from the first liquid chamber. A liquid pressure in the wheel cylinder is controlled so as to put a slip factor of the wheel within a proper range. To such a control of the liquid pressure, the liquid-pressure control piston is moved forward and backward by increasing or decreasing the liquid pressure in the second liquid chamber under control of an electromagnetic liquid-pressure control valve.
In the antilock brake liquid-pressure control apparatus of the variable volume type as described above, a high pressure accumulator and a pressure switch for monitoring a pressure in the accumulator are indispensably used. Additionally, a large size reservoir tank for holding brake oil must be provided in an antilock brake liquid-pressure control circuit. This results in increase of the size and weight of the apparatus. Further, two control electromagnetic valves, a hold valve and a decay valve, must be used for one brake piping system. The accumulator and the pressure switch are expensive, leading to increase of the cost to manufacture the apparatus.